Electrically tunable resistor and related methods

ABSTRACT

An electrically tunable resistor and related methods are disclosed. In one embodiment, the resistor includes a first resistive layer, at least one second resistive layer, and an intermediate interdiffused layer of the first resistive layer and the at least one second resistive layer. One method may include providing a first plurality of layers of different materials surrounded by at least one insulating layer, and passing a current pulse through the first plurality of layers to affect a conductivity structure of the first plurality of layers in order to obtain a first predetermined resistance value for the resistor.

BACKGROUND OF THE DISCLOSURE

1. Technical Field

The disclosure relates generally to integrated circuit (IC) chips, andmore particularly, to an electrically tunable resistor and relatedmethods.

2. Background Art

Thin film resistors are used in the integrated circuit (IC) chipindustry. The resistance R of a rectangular sheet of material of widthW, height H and thickness d is determined by the film resistivity (rho)by: R=rho*W/(H*d). Any of the above mentioned parameters can be variedto vary the resistance R.

A number of challenges exist relative to controlling the resistance.First, if different resistance values in a circuit are desired, thegeometric parameters width W, height H or thickness d, and/or thematerial resistivity rho, must be varied. Second, tolerance variationsin material resistivity rho, width W, height H and thickness d will leadto variations in resistance. Third, new variations in the resistancevalues derived by changing the resistor geometry require new masks tomake specialty IC chips. Finally, changes in the resistance from avariation in material resistivity rho require a change of the depositionmaterial, which is costly and limited by available deposition materials.

One approach to adjust a resistance value includes trimming resistors tochange their physical dimensions. SU 1020869 is an example of thisapproach in which a heating pulse current is applied to the resistorprior to laser trimming.

SUMMARY OF THE DISCLOSURE

An electrically tunable resistor and related methods are disclosed. Inone embodiment, the resistor includes a first resistive layer, at leastone second resistive layer, and an intermediate interdiffused layer ofthe first resistive layer and the at least one second resistive layer.One method may include providing a first plurality of layers ofdifferent materials surrounded by at least one insulating layer, andpassing a current pulse through the first plurality of layers to affecta conductivity structure of the first plurality of layers in order toobtain a first predetermined resistance value for the resistor.

A first aspect of the disclosure provides a method of forming aresistor, the method comprising: providing a first plurality of layersof different materials surrounded by at least one insulating layer; andpassing a current pulse through the first plurality of layers to affecta conductivity structure of the first plurality of layers in order toobtain a first predetermined resistance value for the resistor.

A second aspect of the disclosure provides a resistor comprising: afirst resistive layer; at least one second resistive layer; and anintermediate interdiffused layer of the first resistive layer and the atleast one second resistive layer.

A third aspect of the disclosure provides a method of forming aresistor, the method comprising: providing a first plurality of layersof different materials; first passing a current pulse through the firstplurality of layers to affect a conductivity structure of the firstplurality of layers in order to obtain a first predetermined resistancevalue for the resistor; measuring a resistance of the first plurality oflayers; determining a calibration for tuning the resistance based on themeasuring; providing at least one second plurality of layers ofdifferent materials substantially identical to the first plurality oflayers; and second passing a current pulse through each of the at leastone second plurality of layers to affect a conductivity structure ofeach second plurality of layers in order to obtain a secondpredetermined resistance value different from the first predeterminedresistance value.

The illustrative aspects of the present disclosure are designed to solvethe problems herein described and/or other problems not discussed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of this disclosure will be more readilyunderstood from the following detailed description of the variousaspects of the disclosure taken in conjunction with the accompanyingdrawings that depict various embodiments of the disclosure, in which:

FIG. 1 shows one embodiment of an electrically tunable resistor.

FIG. 2 shows one illustrative tuner for the electrically tunableresistor of FIG. 1.

It is noted that the drawings of the disclosure are not to scale. Thedrawings are intended to depict only typical aspects of the disclosure,and therefore should not be considered as limiting the scope of thedisclosure. In the drawings, like numbering represents like elementsbetween the drawings.

DETAILED DESCRIPTION

Referring to FIG. 1, one embodiment includes a method of forming aresistor 100. First, a plurality of layers 102 of different materials isprovided. As shown, plurality of layers 102 includes a first resistivelayer 104 and at least one second resistive layer 106. Any number ofsecond resistive layer(s) 106 may be employed. Resistive layers 104, 106may include any now known or later developed resistive material.However, in one embodiment, resistive layers 104, 106 each include aconductive material such as a metal. Where metals are used, resistivelayers 104, 106 of, for example, iron (Fe) and cobalt (Co),respectively, may be employed. First resistive layer 104 may include afirst metal material and at least one second resistive layer 106 mayinclude at least one other different metal material. Other materials mayalso be employed including but not limited to chromium (Cr), copper(Cu), silver (Au), gold (Ag), aluminum (Al), ruthenium (Ru), platinum(PI), tantalum (Ta), molybdenum (Mb), tungsten (W) and nickel (Ni).Alloys of the metals could also be used, such as 80 permalloy (80:20Ni:Fe). It is best to choose metals which will mix and interdiffuse withone another during tuning to form a (new) interdiffused layer 108 aswell as having different enough resistivities so that the resistance ofthe fully mixed metals is different enough from the resistance of thetwo initial parallel sheets of the metals. The metals chosen shouldundergo interdiffusion prior to electromigration for the range ofpossible pulse durations and power levels which could be reasonably usedto tune the system resistance. Note the positioning of each layer isonly illustrative. Also, note that interdiffused layer 108 could expandto include the physical extant of layers 104 and 106 after the tuningprocess is complete.

Resistive layers 104 and 106 are positioned between insulating layers110 and 116. That is, insulating layers 110, 116 are adjacent toresistive layers 104, 106, respectively. In one embodiment, insulatinglayer 110, 116 are diffusion barrier layers to ensure interdiffusion ofresistive layers 104, 106 during the tuning pulsing. In anotherembodiment, insulating layers 110, 116 do not prevent diffusion, i.e.,they are chosen so that diffusion into those layers is preferentialduring the tuning pulsing. Note, they could also be chosen to beopposite in terms of their diffusivity. Insulating layers 110, 116 serveto electrically isolate resistive layers 104, 106, and in oneembodiment, to serve as a diffusion barrier to contain the totalthickness of resistive layers 104, 106. Examples of insulating layers110, 116 include alumina, silica and low dielectric constant materials.For electrical isolation, the materials for insulating layers 110, 116are chosen to have a high electrical resistivity relative to resistivelayers 104, 106. For a diffusion barrier, insulator layers 110, 116 arechosen such that resistive layers 104, 106 have a low solubility inthem, such that insulator layers 110, 116 are stable at the temperaturesand pulse durations used to tune resistor 100.

Next, in order to electrically tune resistor 100, a tuner 120 passes acurrent pulse through plurality of layers 102 to affect a conductivitystructure of plurality of layers 102 in order to obtain a firstpredetermined resistance value R1 for resistor 100. That is, tuner 120can alter the resistance of plurality of layers 102 (i.e., resistor 100)by passing current pulse(s) through layers 102 to heat the stack to adesired temperature for a desired pulse duration using an appropriatecurrent level. The desired change in resistance dictates the powerlevel, duration and number of the current pulse(s).

As shown in FIG. 1, the conductivity structure change may result in aninterdiffused layer 108 of first resistive layer 104 and the at leastone second resistive layer 106. For example, where iron (Fe) and cobalt(Co) are used in resistive layers 104, 106, an iron-cobalt alloyinterdiffused layer 108 results. The materials in plurality of layers102 are chosen such that with the proper amount and duration of currentpulse (power applied), interdiffusion occurs in layers 102, resulting ina stable alloy rather than electromigration. In any event, the finalresistance values ranges from the parallel resistance of the separatelayers 104, 106, 108 to the resistance for a complete alloy. Again, notethat depending on the materials chosen and the power applied, layers 104and 106 may have been more or less enveloped by layer 108.

Tuner 120 may further be employed to pass a plurality of current pulsesthrough plurality of layers 102, and measure a resistance change ΔR inplurality of layers 102 after each pulse or group of pulses. Eachcurrent pulse may have a different voltage and/or duration. In anyevent, based on the measurements, a calibration for tuning theresistance can be determined, e.g., for each type of plurality of layersand/or for each layer. For example, layers 102 can be subjected toincreasing current level pulses of controlled levels. The resistancechange after each current pulse can be measured and based on the changein resistance. The next current pulse can either be of a slightly highervoltage or at the same voltage. Current pulse(s) are applied to layers102 until the desired resistance is achieved.

A temperature versus power of the combined initial resistance ofresistor 100 can be determined by the following means. First, theresistance versus temperature of the initial resistor 100 can bemeasured in an oven. Second, the temperature rise (ΔT) versus power(P_(in)) at a fixed pulse duration (τ) is given by the thermalconductance κ_(r), (κ_(r)(τ)=P_(in)/ΔT(τ)) and can be determined forfixed pulse durations by measuring the voltage across, and the currentinto, resistor 100 at low power levels where the sensor is not damaged.The power is given by the current times the voltage, and the resistanceis given by the voltage divided by the current. The temperature rise isthen determined by the measured resistance change, which is here assumedto increase linearly with temperature. The temperature of resistor 100is then determined by assuming a linear relationship between resistor100 resistance and the temperature rise:R_(r)(ΔT(τ))=R_(r)(0)*(1+αΔT(τ)), where R_(r) is the resistance, ΔT istemperature rise, τ is duration and α is a thermal coefficient ofresistance. The thermal coefficient of resistance (α) is measured in anoven. When applying a high-current pulse, the temperature rise is thendetermined by: ΔT(τ)=(P_(in)/κ_(r)(τ)), where ΔT is temperature rise, τis duration, P_(in) is power, and κ_(r) is the thermal conductance.These equations can be used to accurately predict the current/voltagerequirements for tuning resistor 100. Based on this, the temperature ofresistor 100 versus power can also be determined from low voltagepulses.

Based on this information, any number of other substantially identical(e.g., substantially similar dimensions and/or materials) plurality oflayers 130 can also be tuned. In this setting, plurality of layers 102may act as a test section 127 within an integrated circuit (IC) chip125. That is, one set of layers 102 may act as sacrificial parts toquantify the behavior of layers 102 so the appropriate current pulsesfor a given resistance change can be pre-determined. As a result, atleast one second plurality of layers 130 substantially identical toplurality of layers 102 may be provided in IC chip 125 to provideresistor(s) 100. Tuner 120 can then be used to pass a current pulsethrough each of the at least one second plurality of layers 130 toaffect a conductivity structure of each second plurality of layers 130in order to obtain a second predetermined resistance value differentfrom the first predetermined resistance value for plurality of layers102.

The above-described process may occur during manufacture or in-the-fieldafter manufacture. For example, tuner 120 may be employed to alter theresistance of any resistor 100 by passing a current pulse having a knownvoltage through resistor 100 to affect a known change in the resistanceof resistor 100. In-situ resistance tuning enables the manufacturer toloosen the tolerances during deposition of plurality of layers 102, 130.It also allows the manufacturer to use a single deposition process withsubsequent tuning of specific resistors to the desired values.Neighboring resistors made with the same materials can be tuned todifferent values. Based on this structure, resistor networks may beemployed in which series and/or parallel combinations are used toachieve, first, a gross value of resistance and then electrically tunedto the final value. Note, a multiplexer (not shown) may be necessary toreconfigure the resistor network before invoking the electrical finetuning. Furthermore, tuning during the life of the IC chip is nowpossible, further enabling autonomic or self-correcting computing.

A resistor 100 according to one embodiment may include first resistivelayer 104, at least one second resistive layer 106 surrounded by atleast one insulating layer 110, 116, and intermediate interdiffusedlayer 108 of first resistive layer 104 and at least one second resistivelayer 106. Further, resistor 100 may include tuner 120 for passing acurrent pulse through plurality of layers 102 to affect a conductivitystructure of resistive layer 104 and second resistive layer(s) 106 inorder to obtain a first predetermined resistance value for resistor 100.As indicated above, tuner 120 may be operable after manufacture ofresistor 100.

Referring to FIG. 2, one illustrative tuner 120 is shown. The simplestembodiment of tuner 120 is just a single tuner and application circuit132 a. In all cases, the tuning supply could be either internal orexternal to the chip.

Another embodiment would include tuning and application circuits 132a-132 d and a decode circuitry 140. This example is for on-chip tuningand uses decode circuitry 140 to allow each resistor 100 to beindividually tuned. Although simple logic gates are shown and wouldcorrectly function as shown, other schemes, could be used that wouldpotentially save space or write time. Transistor T2 is a decodingactivation/isolation transistor, and transistors T3 and T4 areactivation/isolation transistors from an application circuit 142. Oneimplementation would be for a single set of decoders to be used toselect resistor 100 being tuned and to provide the appropriate tuningvoltage level and pulses externally.

Another implementation would be to use additional decoder circuitry 140,once a given resistor 100 has been selected, to build the appropriatesignal internally. In this case, shown as a fine current tuning circuit150, one or more transistors T6-T8, would be placed in parallel with adecode transistor T5, and used with a fine current tuning decoder 140 ato apply the required signal level. Each transistor could have differingload resistors R2-R5, to apply the appropriate pulse controlled by thesecond decode network, i.e., fine current tuning decoder 140 a.

Each general application circuit 142 type would need to be consideredfor the best tuning circuit, although most would likely be very similar.Application circuit 142 is shown on the other side ofactivation/isolation transistors T3 and T4 when those transistors areused, but would be directly connected to resistor 100 when they are notused. Activation/isolation transistors T3 and T4 are optional isolationtransistors and are off during the tuning current pulse.Activation/isolation transistors T3 and T4 might not be necessary ifapplication circuit 142 has an input impedance that is sufficientlylarge so that application circuit 142 is not damaged by the tuningvoltage pulse. Activation/isolation transistor T2 and decode transistorT5 between resistor R2 and ground is also potentially optional dependingon application circuit 142.

One illustrative operation of tuner 120 would be as follows: First,activation/isolation transistors T3 and T4 would be turned off toisolate resistor 100 from the application circuit 142. Second, finecurrent tuning circuitry 150 would be activated, uniquely turning ondecode transistor T5. Finally, decode circuitry 140 would turn onactivation/isolation transistors T1 and T2 the appropriate length oftime to provide the tuning current pulse. Also note thatactivation/isolation transistor T2 could be operated independently ofactivation/isolation transistor T1. One case would be foractivation/isolation transistor T1 to be turned on and then useactivation/isolation transistor T2 to control the pulse duration.

The above-described circuit and the detailed operations are only meantas an example of an implementation where most of the control is at thechip level. Depending on the circuit and conditions, several turn on andturn off schemes could be envisioned even for this simple circuit.

In principle this device could be used at the end of manufacturingprocess of a first metal layer (M1), at final wafer test, at modulefinal test, or in the field with a system designed for autonomiccomputing. Note that in the first two cases, the tuning of each resistor100 might be best accomplished by addressing each resistor with anexternal tuning supply by means of wafer probing. For the latter twocases, some on-chip decoding would likely be necessary.

Real-time tunable resistor 100 also allows design of a system capable ofdetecting when a given critical parameter is reaching its failure point.In this case, tuner 120 may implement the above-described method to takecorrective action such that: 1) the IC chip or a portion thereof istaken out of service, 2) the tuning activation/isolation transistors areactivated, 3) the appropriate decode circuitry 140 and/or fine currenttuning circuitry 150 is activated, and 4) resistor 100 is then tuned toextend the operating life of that critical component. Although for sometechnology mechanisms, other means are available to extend the life dueto reliability, an electrically tunable resistor 100 provides theability and opportunity to change the operating point of transistorseven for those mechanisms which cannot be directly healed and henceprovide relief during the operation in the field.

Although one illustrative tuner 120 has been shown, it is understoodthat a large variety of other tuners may be employed and are consideredwithin the scope of the disclosure.

The structures and methods as described above are used in thefabrication of integrated circuit chips. For example, resistor 100 maybe used in a tape head, magnetic random access memory (MRAM), etc. Theresulting integrated circuit chips can be distributed by the fabricatorin raw wafer form (that is, as a single wafer that has multipleunpackaged chips), as a bare die, or in a packaged form. In the lattercase the chip is mounted in a single chip package (such as a plasticcarrier, with leads that are affixed to a motherboard or other higherlevel carrier) or in a multichip package (such as a ceramic carrier thathas either or both surface interconnections or buried interconnections).In any case the chip is then integrated with other chips, discretecircuit elements, and/or other signal processing devices as part ofeither (a) an intermediate product, such as a motherboard, or (b) an endproduct. The end product can be any product that includes integratedcircuit chips, ranging from toys and other low-end applications toadvanced computer products having a display, a keyboard or other inputdevice, and a central processor.

The foregoing description of various aspects of the disclosure has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the disclosure to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to aperson skilled in the art are intended to be included within the scopeof the disclosure as defined by the accompanying claims.

1. A method of forming a resistor, the method comprising: providing afirst plurality of layers of different materials between two insulatinglayers; passing a current pulse through the first plurality of layersfor a first predetermined pulse duration to attain a first predeterminedtemperature to affect a conductivity structure of the first plurality oflayers in order to obtain a first predetermined resistance value for theresistor; providing at least one second plurality of layers of differentmaterials substantially identical to the first plurality of layers;passing a current pulse through each of the at least one secondplurality of layers for a second predetermined pulse duration to attaina second predetermined temperature to affect a conductivity structure ofeach second plurality of layers in order to obtain a secondpredetermined resistance value different from the first predeterminedresistance value; and repeating the passing of the current pulsesthrough both of the first plurality of layers and the at least onesecond plurality of layers until each has attained a desired resistancevalue.
 2. The method of claim 1, wherein the first plurality of layersincludes a cobalt (Co) alloy layer and a nickel: iron (Ni:Fe) alloylayer.
 3. The method of claim 1, wherein each of the first plurality oflayers includes a conductive material.
 4. The method of claim 1, furthercomprising measuring a resistance change in the first plurality oflayers after each pass of the current pulse.
 5. The method of claim 4,wherein each current pulse has at least one of a different voltage and adifferent duration.
 6. The method of claim 4, further comprisingdetermining a calibration for tuning the resistance of the firstplurality of layers based on the measuring of the resistance change inthe first plurality of layers after each pass of the current pulse. 7.The method of claim 1, wherein the first plurality of layers includes atest section within an integrated circuit (IC) chip.
 8. The method ofclaim 1, further comprising measuring a resistance change in of the atleast one second plurality of layers after each pass of the currentpulse.
 9. The method of claim 8, further comprising determining acalibration for tuning the resistance of the at least one secondplurality of layers based on the measuring of the resistance change inthe at least one second plurality of layers after each pass of thecurrent pulse.
 10. The method of claim 1, wherein the insulating layersare diffusion barrier layers.
 11. A method of forming a resistor, themethod comprising: providing a first plurality of layers of differentmaterials; first, passing a current pulse through the first plurality oflayers to affect a conductivity structure of the first plurality oflayers in order to obtain a first predetermined resistance value for theresistor; measuring a resistance of the first plurality of layers afterpassing the current pulse therethrough; repeating the passing of thecurrent pulse through the first plurality of layers and measuring of theresistance of the first plurality of layers after passing a currentpulse; determining a calibration for tuning the resistance of the firstplurality of layers based on the measuring of the resistance change inthe first plurality of layers after each pass of the current pulse;providing at least one second plurality of layers of different materialssubstantially identical to the first plurality of layers; second,passing a current pulse through each of the at least one secondplurality of layers to affect a conductivity structure of each secondplurality of layers in order to obtain a second predetermined resistancevalue different from the first predetermined resistance value; measuringa resistance of the at least one second plurality of layers afterpassing the current pulse therethrough; repeating the passing of thecurrent pulse through the at least one second plurality of layers andmeasuring of the resistance of the at least one second plurality oflayers after passing a current pulse; determining a calibration fortuning the resistance of the at least one second plurality of layersbased on the measuring of the resistance change in the at least onesecond plurality of layers after each pass of the current pulse; andrepeating the passing of the current pulse through the first and the atleast one second plurality of layers that is in accordance with eachrespective calibration, measuring and determining a calibration aftereach passing of a current pulse until a desired resistance value hasbeen attained for the first plurality of layers and the at least onesecond plurality of layers.
 12. The method of claim 11, wherein eachplurality of layers includes a metal.
 13. The method of claim 11,wherein each current pulse has at least one of a different voltage and adifferent duration.
 14. The method of claim 11, wherein the firstplurality of layers includes a test section within an integrated circuit(IC) chip.